Messages in this thread | | | Date | Tue, 9 Jun 2020 17:45:06 +0530 | From | afzal mohammed <> | Subject | Re: ARM: vmsplit 4g/4g |
| |
Hi,
On Mon, Jun 08, 2020 at 08:47:27PM +0530, afzal mohammed wrote: > On Mon, Jun 08, 2020 at 04:43:57PM +0200, Arnd Bergmann wrote:
> > There is another difference: get_user_pages_fast() does not return > > a vm_area_struct pointer, which is where you would check the access > > permissions. I suppose those pointers could not be returned to callers > > that don't already hold the mmap_sem. > > Ok, thanks for the details, i need to familiarize better with mm.
i was & now more confused w.r.t checking access permission using vm_area_struct to deny write on a read only user page.
i have been using get_user_pages_fast() w/ FOLL_WRITE in copy_to_user. Isn't that sufficient ?, afaiu, get_user_pages_fast() will ensure that w/ FOLL_WRITE, pte has write permission, else no struct page * is handed back to the caller.
One of the simplified path which could be relevant in the majority of the cases that i figured out as follows,
get_user_pages_fast internal_user_pages_fast gup_pgd_range [ no mmap_sem acquire path] gup_p4d_range gup_pud_range gup_pmd_range gup_pte_range if (!pte_access_permitted(pte, flags & FOLL_WRITE)) [ causes to return NULL page if access violation ]
__gup_longterm_unlocked [ mmap_sem acquire path] get_user_pages_unlocked __get_user_pages_locked __get_user_pages follow_page_mask follow_p4d_mask follow_pud_mask follow_pmd_mask follow_page_pte if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) [ causes to return NULL page if access violation ]
As far as i could see none of the get_user_pages() caller are passing struct vm_area_struct ** to get it populated.
And Ingo's series eons ago didn't either pass it or check permission using it (it was passing a 'write' arguement, which i believe corrresponds to FOLL_WRITE)
Am i missing something or wrong in the analysis ?
Regards afzal
| |